There is an air-conditioning apparatus in which a heat source unit (outdoor unit) is arranged outside a construction and indoor units are arranged inside the construction, such as a multi-air-conditioning device for buildings. A refrigerant circulating in a refrigerant circuit in such an air-conditioning apparatus transfers (removes) heat to (from) air to be supplied to a heat exchanger in an indoor unit, thereby heating or cooling the air. The heated or cooled air is sent to an air-conditioned space so that the space is heated or cooled.
As a heat source side refrigerant for use in such an air-conditioning apparatus, a hydrofluorocarbon (HFC)-based refrigerant is used in many cases. As the heat source side refrigerant, a refrigerant using a natural refrigerant, such as carbon dioxide (CO2), has also been proposed.
As an air-conditioner, one configured to include a plurality of indoor units, each of which is capable of selecting heating operation or cooling operation is proposed (see, for example, Patent Literature 1). The technique described in Patent Literature 1 has a cooling only mode, in which all indoor units perform cooling operation, a heating only mode, in which all indoor units perform heating operation, a heating main mode in simultaneous cooling and heating as simultaneous cooling and heating operation with the larger heating load, and a cooling main mode in simultaneous cooling and heating as simultaneous cooling and heating operation with a larger cooling load. The technique described in Patent Literature 1 switches between the heating only mode and the heating main mode in simultaneous cooling and heating or between the cooling only mode and the cooling main mode in simultaneous cooling and heating by switching one of a plurality of four-way valves.
There also exists an air-conditioning apparatus having another configuration typified by a chiller system. In such an air-conditioning apparatus, a heat source unit arranged outside a room generates cooling energy or heating energy, a heat exchanger arranged inside an outdoor unit heats or cools a heat medium, such as water or an antifreeze solution, the heat medium is transported to a fan coil unit, a panel heater, or the like that is an indoor unit arranged in an air-conditioned space, and cooling or heating is performed (see, for example, Patent Literature 2).
There also exists a heat source side heat exchanger called an exhaust heat recovery chiller in which four water pipes are connected between a heat source unit and an indoor unit, cooled water and heated water and the like are simultaneously supplied, and cooling or heating can be freely selected in the indoor unit (see, for example, Patent Literature 3).
There also exists an air-conditioning apparatus in which a heat exchanger for a primary refrigerant and a heat exchanger for a secondary refrigerant are arranged in the vicinity of each indoor unit and the secondary refrigerant is transported to the indoor unit (see, for example, Patent Literature 4).
There also exists an air-conditioning apparatus in which two pipes are connected between an outdoor unit and a branch unit including a heat exchanger and a secondary refrigerant is transported to an indoor unit (see, for example, Patent Literature 5).